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1. (Original post by SeanFM)
P(Z<1.96) = 0.975.

Remember that with an a value when we want to find P(X<a) we do (a-mean)/standard deviation to turn a into z, and find P(Z<z).

If a was 18 for example and we wanted to find P(X<18) then we would do P(Z<(18-15)/4) = P(Z<3/4).

What I was trying to hint at was that each value for a has a value for z that we calcuate, and in that example (a-15)/4 = 3/4, and in your question it's ..... = 1.96.
so
a-15 / 4 = 1.96
multiply by 4
a-15 = 7.84
a = 22.84 ??
2. (Original post by Aty100)
so
a-15 / 4 = 1.96
multiply by 4
a-15 = 7.84
a = 22.84 ??
Yes, well done

If you can now see what I meant by 'working backwards' then you have done very well.
3. (Original post by SeanFM)
Yes, well done

If you can now see what I meant by 'working backwards' then you have done very well.
FINALLY !

Yess thanks soo much for your time !
4. (Original post by SeanFM)
Yes, well done

If you can now see what I meant by 'working backwards' then you have done very well.
so for
X~N ( 85 , 25)
P ( X > a ) = 0.67

would you do 1 - 0.67 because it is greater than and that is 0.33

a - 85 / 5 = 0.33 ?
a - 85 = 1.65
a = 85.65
5. (Original post by Aty100)
so for
X~N ( 85 , 25)
P ( X > a ) = 0.67

would you do 1 - 0.67 because it is greater than and that is 0.33

a - 85 / 5 = 0.33 ?
a - 85 = 1.65
a = 85.65
Remember that you need to equate the transformation (Which is (a-mean)/standard deviation) to a z value, not the actual probability.
6. (Original post by SeanFM)
Remember that you need to equate the transformation (Which is (a-mean)/standard deviation) to a z value, not the actual probability.
so would I equal to 0.6293 ?
7. (Original post by Aty100)
so would I equal to 0.6293 ?
If that's the correct value from the tables, then yes.
8. (Original post by SeanFM)
If that's the correct value from the tables, then yes.
You're soo awesome! Thanks once again for your time and help. I appreciate it !
9. (Original post by Aty100)
You're soo awesome! Thanks once again for your time and help. I appreciate it !
No problem

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